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Keys to History

Two types of coffer dams were eventually used for building the piers. Initially, floating coffer dams, invented by James Hodges, were employed. Two were built above the bridge site in the winter of 1853-1854. In the spring each dam was floated down to the pier site and positioned by one of the steam tugs. Once properly aligned, it was sunk to the river bed by opening the "scuttling valves".

The main advantage of the floating dam was that it could be re-used after the pier was completed: the dam was pumped out, a detachable section in the stern removed and the dam towed away to the next site or to winter storage at Sorel. Its main disadvantage was a serious one: where the river-bed was irregular, it was difficult and at times impossible to achieve a good seal to keep the water out of the work chamber. Besides, a floating dam could not be used on any pier that would not be finished before freeze-up. Nevertheless, when conditions were ideal it could be set up in only four days, rather than the several weeks needed to install alternate systems. Floating dams were used to build five of the piers. The dams for two additional piers spanning the main channel of the river as well as the north abutment were formed with a combination of floating dams and cribwork.